Sonic the Hedgehog 3 was really at the pinnacle of gaming when it was first released for the Sega Genesis/Mega Drive. It really had a lot to prove since its two predecessors were extremely successful but if anything, Sonic 3 is now considered (along with Sonic and Knuckles) one of the best Sonic games ever released.
Sonic 3 kept the true sense of what a Sonic game should be about and that is speed - the game has lots of it to offer with it's blisteringly fast action which require top notch reflexes, but it also allows you to run through at your own pace should you so wish. Tails was included as he was in Sonic 2 but now with the ability to use his fly ability and Knuckles (a red echidna who is unplayable in Sonic 3 alone except in the multiplayer races) was introduced to gamers for the first time.
When the sequel Sonic and Knuckles came out with added lock-on technology, it meant gamers could attach the two games for twice the gameplay and a lot of cool new features.
The gameplay is pretty much the same as the previous Sonic the Hedgehog games with the ability to run and jump, spindash and collect shields and rings. However different shields give Sonic different powers, for example the bubble allowed him to bounce and breathe underwater, the lightning shield protected from certain projectiles, electricity and also attracted nearby rings and the fire shield also protected from certain projectiles, fire and allowed Sonic to use a forwards mid-air dash attack.
The game has 7 stages in all each with 2 acts, each act takes place in a different environment ranging from jungles to carnivals, underwater cities to icy baron lands. Act 1 always has a sub-boss which isn't Dr Robotnik - Act 2 you always have to fight Dr Robotnik so they did away with the whole 1 boss per level thing. But the sub-bosses are never that hard.
The Bonus Stages include a 3D maze of red and blue orbs which Sonic must navigate collecting only the blue orbs in order to obtain a Chaos Emerald. The other being a smaller gumball machine which allowed Sonic to earn shields and extra lives.
Super Sonic is obtainable by collecting all 7 Chaos Emeralds. Unfortunately in Sonic 3 alone, Tails cannot gain any power from collecting all the Emeralds.
Multiplayer and Online Capabilities
This game has a Leaderboard as do most if not all of the Genesis/Mega Drive ports do, however due to an exploit on the last level which allows for the infinite respawn of badniks which meant gamers could boost their score up to stupidly high figures, the leaderboards will probably not reflect how good a gamer is.
The game has Offline Co-op similar to Sonic 2, with both Sonic and Tails both playable at one time during the main story, however the camera only focuses on Sonic meaning Tails is always left behind - but he is invincible to an extent.
It also has online and offline multiplayer races - allowing two players to race each other doing laps on a set of small tracks. Each track is unique from any of the story mode levels in terms of sprites and music. Players could play as Sonic, Tails or Knuckles in this mode but the latter do not have their fly/glide abilities.
In Terms of Achievements, it has the easiest 200G out of all 3 of the Sonic games for XBLA so far, almost half of the achievements can be obtained on the first two level. The rest aren't too hard either and can be earnt in one playthrough, the hardest achievement probably being the collection of the 7 Chaos Emeralds. However as with all of the Genesis/Mega Drive games on XBLA, you can save and load anywhere in the game at any time, so saving the game frequently to avoid failure is a sure way to make it easier.
If you are a hardcore Sonic and Genesis/Mega Drive fan then no doubt you will already have your heart set on this game, if your not and your looking for an easy 200G, I recommend this game. It only costs 400MSP which is a bonus!
I think these Sonic games really give a hint of nostalgia to older gamers, too bad that todays younger generation don't see the fun and exciting gameplay these type of games had to offer back in the day.